TLEdata

TLEdata, or two-line element data, is a compact textual format used to convey the orbital parameters of Earth-orbiting satellites. It provides a snapshot of a satellite's orbit at a given epoch and is widely used for rapid propagation of satellite positions using standard orbital models.

A complete TLE entry consists of two lines of ASCII text. Each line begins with a line number, 1 for the first line and 2 for the second. The two lines together encode the satellite identity, epoch, drag terms, and a set of orbital elements that describe shape and orientation of the orbit.

The first line contains the satellite catalog number, international designator, epoch time, and terms related to orbital drag and derivative coefficients. The second line carries the core orbital elements: inclination (tilt of the orbit), right ascension of the ascending node, eccentricity, argument of perigee, mean anomaly, and mean motion, along with the revolution number at epoch and a checksum.

TLEdata are produced by orbital surveillance networks and distributed by public repositories such as Celestrak and Space-Track. They are updated frequently to reflect new tracking data and occasional reclassifications, and are used to initialize orbital propagators such as SGP4/SDP4. Applications span satellite tracking, mission analysis, and educational visualization.

Limitations include the approximate nature of TLEs and their relatively short predictive horizon. They are most reliable within days to a few weeks of the epoch and can degrade for highly perturbed or high-drag environments. TLEdata remain a practical standard for quick, interoperable orbit representation and widely cited in the space domain.